Page 231 - Geology of Carbonate Reservoirs

212 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

wave base characterize nearshore zones on ramps and slope breaks on shallow
shelves, but similar environments occur on and around bathymetric highs, or shoals,
surrounded by deep water. For example, salt domes, horsts, erosional remnants, and
preexisting highs such as drowned dunes or reefs may have been shallow enough
for waves and currents to sweep across their crests constantly under normal weather
conditions, setting the stage for deposition of grainstones or reefs that ordinarily
form along shorelines or shelf - edge slope breaks. There are many examples of
shallow - water deposits on tops of antecedent highs surrounded by deeper water
(Ahr, 1989 ).
Debrites, turbidites, and deep - water mudstone – cementstone buildups may not be
easy to locate even if the platform type and the position of the strandline are known.
Abundant subsurface geological data will provide the best clues to the location of

debrites and turbidites that would otherwise be difficult to identify on seismic pro-

files unless the deposits were more than about 50 feet thick (rough estimate of
one - quarter wavelength of a seismic impulse) and were surrounded by different
rock types with enough seismic contrast to be recorded by the seismograph. Many
mud and cementstone buildups have been detected with the seismograph but deter-
mining which ones have reservoir - quality porosity and permeability is very diffi cult
until they are tested by the drill or until unique seismic attributes can be extracted
to identify potential reservoirs. Many Early Carboniferous mudstone – cementstone
buildups are located on the toe of slope in outer ramp environments in basins
around Europe and North America (Ahr, 1998 ). Many of these examples tend to
nucleate in transgressive systems tracts (TST) and fully develop during high stands
(HST).

Interpreting depositional reservoirs, or confirming the identity of successions
penetrated by the drill, is mainly a matter of comparing cored intervals with the
seven standard successions discussed in Chapter 5 . Sometimes it is possible to iden-
tify depositional successions from detailed cuttings logs, but the most reliable and
satisfactory work depends on core descriptions. Grainstones and packstones are
common on strandplains where dune and beach or barrier island successions are

deposited. Those facies are recognizable by their vertical profile of textures, grain
types, sedimentary structures, and fossil content, as discussed in Chapter 5 . Grain -
supported rocks also occur on tops of antecedent highs, around patch reefs on ramps
and shelf interiors, and along slope breaks at shelf edges. These deposits lack the
upper shoreface and dune segments that occur on beaches and barrier islands
because deposits along slope breaks and around paleo - highs are “ always wet. ” That
is, there are few, if any, beach and dune deposits around paleo - highs and shelf - edge
slope breaks. The anatomy of these deposits is determined by, or limited by, the
shape of the bathymetry on which they were deposited, not by the zero isobath of
the strandplain. Tidal bars, slope - break deposits such as those on the Great Bahama

Bank, are confined to the shelf edge where they occur as a series of lobate sand

fingers lying perpendicular to the shelf edge near the drop - off to abyssal depths.
Inboard from the sand waves is a shallow, mud - dominated subtidal environment.
Similarly, grainstones and packstones on paleo - highs and around patch reefs occur
on tops or perimeters of the highs. They also lack beach and dune sedimentary
structures, they are not elongate parallel to a pinchout edge, and they may be sur-
rounded by deeper subtidal environments that have distinctive, deep subtidal fossil
assemblages.